1. Technical Field
The present disclosure relates to liquid crystal displaying technologies, and particularly, to a circuit for eliminating threshold voltage differences between backlight LED strings and a liquid crystal display using the same.
2. Description of Related Art
Due to advantages of light emitting diodes (LEDs) such as long lasting, energy saving, and easily to be driven, LEDs are widely used in various electronic devices, such as backlight modules of liquid crystal displays (LCDs). The backlight module generally includes edge-lighting type backlight modules and direct type backlight modules. In the direct type backlight modules, the LEDs are located under the liquid crystal panel. In the edge-lighting type of the backlight modules, a number of LED strings are disposed around a liquid crystal panel of the LCD. Each of the LED string includes a number of LEDs connected in series.
In the manufacturing process of the LED backlight modules, the LEDs are connected in series to from the LED strings respectively. Due to the different manufacturing technology, threshold voltages of the LED strings may be different from each other to result in a threshold voltage difference between two LED strings. This may further result in energy consumptions on MOS transistors respectively connected to the LED strings and further increases of the temperatures of the MOS transistors. In this state, not only the performance of the MOS transistors are influenced, but also constant current integrated circuits used for respectively controlling the LED strings are heated to consume energy, which reduces efficiency of the backlight module.
Generally, a current of each of the LED strings can be increased and an on state duration t or a duty ratio D of each of the LED strings can be reduced to reduce the voltage differences between the LED strings.
To a first LED string, according to the calculating formula of the duty ratio D: D=t/T=t*F, (wherein t refers to the on state duration, T refers to an on state period, and F refers to a threshold frequency), when the threshold voltage (labeled as Vf) equal to 40V, the current I is equal to 10 mA, the on state duration t is equal to 1 s, and the on state period T is equal to 10 s, the duty ratio D of the first LED string can be calculated to be 10% and a threshold energy for turning on the first LED string reaches 0.4 J according to the following expression: W=Vf*I*D*T=0.4 J.
Due to the manufacturing difference between the LED strings, the threshold energy for turning on a second LED string can reach W=0.75 J when the threshold voltage Vf thereof is equal to 50V, the current I thereof is equal to 15 mA, and the duty ratio D is the same as that of the first LED string. Therefore, the threshold energies of the two LED strings are different from each other and the brightness of the two LED strings may become different from each other.
In other situations, the on state duration t may be changed to adjust the duty ratio D to allow the threshold energy of the second LED string to reach 0.4 J, thus, the threshold energy of the second LED string can keep the same as that of the first LED string.
However, the on state duration t cannot be reduced unlimitedly due to the interior structure of the constant current integrated circuit, therefore, the energy consumption of each of the LED strings cannot be reduced unlimitedly.
Therefore, there is room for improvement in the art.